Most industrial Ac motors in the 500 horsepower to 10,000 horsepower range utilize ducted rotors and stators. Such ducted rotors are ones which have longitudinally extending vent ducts inside the rotor and also one or more duct sections wherein air passageways extend radially outwardly between the squirrel cage rotor bars to the periphery of the rotor. Usually, the stator has aligned, radially directed ducts to accept such air flow. The air flow is created by the inherent pumping action of the radial passageways at the rotor periphery, being of a larger radius than the inlets to the longitudinal vent ducts. Additionally, one or more fans may be provided on the rotor to enhance the air flow for cooling.
The prior art ducted rotors using a cast squirrel cage, e.g., of aluminum, were generally cast under high pressure in order to attempt to fill the mold cavity with aluminum as rapidly as possible for high production rate and to avoid excessive heating of the dies. Such high pressure casting utilized pressures in the order of 500 to 2000 psi for injection and then about 1000 psi during the consolidation phase, namely, the pressure on the molten aluminum during cooling, until it solidified sufficiently to permit removal from the casting press. To make a ducted rotor under this system, it was usual, as shown in U.S. Pat. Nos. 3,182,949 or 3,166,804, to utilize hardened steel finger inserts and split clamping rings. These finger inserts had to be inserted by hand during the lamination stack assembly prior to squirrel cage casting. Also, such finger inserts had to be removed by hand after casting. The tooling costs for each different rotor diameter and for each different type of conductor bar slot were quite expensive. Additionally, six to eight hours of labor were required to insert and remove such finger insert tooling. Furthermore, the chance of improper duct tooling placement in a rotor with 200 to 1000 separate finger insert tools was great, and if there was such improper placement, then this could result in molten aluminum leaks, which could cause an incomplete cavity fill, open conductor bars, and unusable rotors. High pressure cast ducted rotors experienced a 5%-10% scrap rate.
A second ducted rotor method of the prior art was the use of a flat metal stock which was formed into an elongated loop as a conductor bar spacer and then spot-welded to a rotor lamination. These conductor bar spacers formed part of the mold cavity in the duct section of the rotor. Such bar spacers were more susceptible to aluminum leaks during casting, due to distortions introduced by the spot welding and by the high clamping forces inherent in the high pressure process.
Still another method of making ducted rotors was to fabricate the squirrel cage. Metal bars, for example of copper, were inserted through the conductor bar apertures in a stack of laminations, and then the ends of the copper bars were brazed to a circular end ring at each end of the stack of laminations. Such fabricated squirrel cage could be used with ducted rotors wherein duct spacer sections were used along the length of the rotor.
A primary difficulty with the high pressure cast aluminum or aluminum alloy squirrel cage rotors was the high scrap rate of 5%-10%, plus considerable voids in the cast metal, especially in the upper end ring and unitary fan blades. Such voids were caused by the turbulence in the flowing metal under high pressure. Also, air trapped within the mold caused such voids in the sprues and end rings. Another problem with the high pressure cast rotors was that the rotor often was not straight, even though the laminations were machined after the rotor was cast. This lack of straightness was due to the fact that the porosity in one group of conductor bars on one side of the rotor might be different from the degree of porosity on the other side of the rotor, and when the rotor heated up during operation distortion resulted. This caused unbalance and vibration.
The fabricated squirrel cages utilizing preformed conductor bars were usually considerably more costly in labor and materials, so were rarely used.
The problem to be solved, therefore, is how to construct a ducted rotor at less cost, using less time and tooling, and with a lower scrap rate of completed rotors.